From NEC Retro

Hardware

While it can be difficult to determine exactly when new "generations" of PC-9800 computers begin, the PC-9801 VM is typically labeled as the most significant PC-9800 series machine, as it was during this computer's run where growth of the format was at its highest. The VM became the de facto "standard" for PC-9800 machines, cementing the use of NEC's V30 processor and typically being the machine which most PC-9800 software was designed for at the time.

The PC-9801 VM exists in multiple forms. In July 1985 consumers could choose between the cheaper "VM0" option which lacked floppy disk drives, the "VM2" variant which included two 5¼-inch drives built into the machine, and a "VM4" machine which threw in a 20MB hard drive. Such was the popularity of this model, NEC released a VM21 setup in November 1986 (which included more RAM) and a cost-reduced VM11 setup two years later.